Various arrangements have been proposed for reading digital data encoded on optically sensitive media. These have included semi-mechanical devices involving steerable mirrors in an optical magnifier disposed between the record frames and optically responsive detection equipment. Such devices are shown, for example, in U.S. Pat. Nos. 4,251,126 to Minoura et al, 3,072,889 to Willcox and, in the context of videodiscs, in U.S. Pat. No. 4,118,735 to Wilkinson. For higher speed readout, an optically based system employing prism optics to project images of multiply coloured record frames onto successive charge storage detectors, is disclosed in U.S. Pat. No. 3,959,784.
The advantages of optically readable storage relative to magnetic tape storage and the like are appreciated: magnetic tapes recorded at 10,000 bits per linear inch, can store 160,000 bits per square inch, while optical media, at 30 square microns per bit, can provide a data density 134 times greater. In situations such as distribution of information, where a read-only facility is appropriate, this increase in data density can provide significant advantage. However, such advantage is presently outweighed by the complexity and costs of adequate registration mechanisms for data of the improved density which can be achieved with optical media. Data storage in this form is simply not presently economical notwithstanding the known advantages potentially available.
The traditional approach to achieving adequate registration between read heads and data has been to ensure sufficient mechanical precision in the location of read heads with respect to the channels in which the record frames are arranged. In the case of magnetic tape storage, for example, the 7 or 9 heads each read their own track, and the tape is aligned mechanically to prevent a read head reading the wrong track. As the density of data increases, the mechanical difficulty of the registration operation also increases and the cost of the required mechanism rises. This is well demonstrated in the case of videodiscs, where a single laser beam is servo guided along a track 1 micron wide. U.S. Pat. No. 3,919,697 to Walker discloses an arrangement in which the data channels or tracks are bordered and separated by "tracking information" to maintain head alignments.
U.S. Pat. No. 3,322,935 to Wyke discloses an optical barcode reader arranged to compensate for misregistration between the read head and the barcode. The technique requires a substantial number of detectors per bar element and each detector is of a type having a simple 2-level output. As described, a chain of such detectors is employed to distinguish elements of immediately adjacent bar lines. One boundary of the pair of bar lines is first located, and a signal characteristic of the position of that boundary is then correlated with overlapping matrices of detector values to select five of the ten values which are most nearly centred on the respective bar elements and which therefore have the best chance to be scanning correct information. The five selected values are then fed to a majority selective matrix from which a reading is output if signals are derived from a majority of the selected detectors according to specified criteria. It will be appreciated that the final pair of output signals are simply selected ones of the detector outputs and that, given the requirement for multiple detectors per data bit and complex circuitry needed to located the barline boundary, the method is not readily adaptable to highly densely stored data.
U.S. Pat. No. 3,795,902 to Russell discloses a recording and playback system in which synchronization is secured for playback by detection of and response to a configuration of digital signal, either from known characteristics of the signal or from information added to the signal during recording. The information thus read out is suitably employed for shifting digital words in a reassembly shift register until proper word synchronization is achieved.
Even once synchronization has been initially achieved, local faults may arise in the regular position indicators or markers by which synchronization is sustained. Many devices currently exist which need to determine the position of a read head relative to data, but most such devices are read/write in that they can record data as well as read it. Any fault during the writing process may result in mispositioned data and the reading process needs to recognize this possibility. Hence it is common for each position determination to override all previous determinations in an absolute fashion, or if any note of previous determinations is made, they serve only to alert the reading system to the presence of an error.
In the case of magnetic discs, for example, the synchronization pattern along the track (whether before the sector number data or on the servo surface) must normally be complete and accurate for the sector to be accessible. Any fault in the pattern generally results in the sector being unusable.
Similarly the next frame on magnetic tape drives must be observed within a specified period since the last or an error is reported, so that a systematic error in frame positioning such as an incorrect density cannot in general be accommodated.
Such position determination processes as used in the conventional computer context may well be best suited to read/write media. When applied to a read only medium whose production is carefully controlled, such as the optically readable media with which this invention is primarily concerned, they suffer excessive sensitivity to faults arising from spot obliteration such as by dust, medium nonuniformity etc. Reverting to the example above, a single bit fault in the synchronization pattern on a magnetic disc is sufficient to render the entire sector which follows unusable. When a medium is subject to a high number of such spot faults, a new approach needs to be taken to ensure accurate position determination so that local faults or "spot errors" in synchronization markers or patterns, or other position indicators, are distinguishable from more serious system deficiencies and do not render whole segments of data unread or unreadable.